Bottom!Optimus is totally a kink in and of itself lol. Seriously though, I felt like this encounter was key in their developing relationship for reasons that you stated already in the story. Ratchet seems to need a solid sense of control, and when you combine that with the fact that Optimus undoubtably gets tired of being the leader everywhere else it actually makes more sense to me that he would let Ratchet dominate him more than he would activly dominate Ratchet. I believe Borath uses the same idealogy in some of her IronhidexOptimus stories. (I don't have an account on here, but I'm Screamtothestars on DA. I don't really like leaving anon comments)